1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, particularly to improvement of a technology of more stably reproducing intermediate gradation.
2. Description of the Related Art
Conventionally, electrophotography is adopted as a process of forming an image on a paper medium in an image forming apparatus such as a copying machine, a printer, a facsimile, and a multifunction peripheral. In the electrophotography, an electrostatic latent image is formed on a photosensitive body (typically, a photosensitive drum or a photosensitive belt) using an exposure device, and the image is formed by developing the electrostatic latent image.
Recently, high resolution of the electrophotography makes progress. For example, the resolution of the electrostatic latent image advances from 600 dpi (dot per inch) to 1200 dpi by improvement of an exposure device. The high resolution of 2400 dpi can be achieved in a high-grade model.
On the other hand, a demand for improving process stability also arises with the improvement of the resolution. Because it is said that the improvement of the resolution is contradictory to the process stability, there is an important technical problem in that the stability is maintained while the resolution is enhanced. The process stability affects the finish of intermediate gradation.
Therefore, for example, Japanese Laid-Open Patent Publication No. 05-161013 discloses a digital recording apparatus that can prevent degradation of image quality caused by an environmental variation, degradation of a density sensor, and degradation of a surface of the photosensitive body and always stably retain the high image quality. Japanese Laid-Open Patent Publication No. 05-328112 discloses a dither process method in which the image having gradation can be restored by performing dither process corresponding to a density state of the image around each pixel constituting a gray scale image even if the target gray scale image has a deviation of the density.
Generally, in the electrophotographic image forming apparatus, the intermediate gradation is reproduced using a halftone technique. In the halftone technique, an objective gradation value is reproduced by controlling a coloring amount (typically, toner adhesion amount) per unit area using a pattern including small dots or lines. In the control of the coloring amount per unit area, plural screens are previously prepared while correlated with plural gradation values, and the screen is selected according to the density to be reproduced. In the general screen, “adhesion regions” that should be colored and “non-adhesion regions” that should not be colored are regularly disposed at a predetermined period. An interval between the “adhesion region” and the “non-adhesion region” can be shortened in the screen with increasing resolution. However, because the process stability can be degraded as described above, there is a restriction to the shortening of the interval between the “adhesion region” and the “non-adhesion region”. As a result, although the electrostatic latent image having the high resolution can be formed, the electrostatic latent image having the high resolution is applied only to a character region, and the screen similar to that of the conventional image forming apparatus having the low resolution is used in the intermediate gradation region.